Conventionally, a plasma cutting apparatus having an electrode, a nozzle disposed as a cover with a spacing for a plasma gas passage, and an electrically insulated protective cap, having an opening facing a nozzle orifice on the outside and at the extreme end of the nozzle, is employed to cut a workpiece while causing a protective gas to flow from the opening of the protective gap to protect the nozzle.
such conventional art will be described with reference to FIG. 5. An electrode 61 is provided at a center of a torch 60, and a nozzle 65 is disposed outside the electrode 61 as a cover with a spacing for a plasma gas passage 63. A high-temperature plasma gas flow 72, formed by causing a plasma gas flow and by simultaneously generating a plasma arc between the electrode 61 and a workpiece 71, is made thinner by the orifice 65a of the nozzle 65 and is jetted to the workpiece 71, thus performing cutting. In this cutting, the shape of the orifice 65a of the nozzle 65, for making the plasma gas flow 72 thinner, largely affects cutting qualities (an inclination of a cut surface, roughness of the cut surface, attachment of dross which is a molten metal, and the like). That is, during the operation of forming a hole in order to be able to start a cutting operation (referred to hereinafter as the time of piercing), if dross 73, blown upwardly from the workpiece 71, attaches to the nozzle 65 or if the nozzle 65 and the workpiece 71 contact each other due to thermal strain or the like during cutting, the orifice 65a is deformed, so that the cut quality is considerably reduced. This is a serious problem in terms of practical use. To cope with this problem, a protective cap 69, having an opening facing the orifice 65 of the nozzle 65, is provided as shown in FIG. 5. This protective cap 69 is electrically insulated from the electrode 61 and the nozzle 65 to prevent irregular discharge (double arc). Also, a protective gas is caused to flow from the cap opening from the start of a cutting to the completion of the cutting, to protect the nozzle 65 against dross 73 blown upwardly from the workpiece 71 (refer to, for example, Japanese Patent Application No. 4-72109).
In this operation of protecting the nozzle with the protective cap, a large amount of protective gas is caused to flow at the time of piercing as well as at the time of cutting. The following two problems are therefore encountered. First, because the flow velocity and the flow rate of the protective gas are large, the high-temperature plasma gas flow downstream of the nozzle is so cooled that the cutting ability is reduced and the amount of attachment of dross is increased, thereby affecting the cut quality. If only a small misalignment occurs between the axes of the orifice of the nozzle and the opening of the protective cap, it also has a bad influence; that is, the plasma gas flow is inclined and disturbed by the strong protective gas flow. Therefore, a special mounting device or a longer mounting time is required in order to achieve a high accuracy of mounting the protective cap.
The second problem is that the operating cost is high because the amount of protective gas used is large. The operating cost is allowable when a low-priced gas such as air is used as the protective gas, but it becomes high when a high-priced gas is used. As a method for externally shielding a plasma gas flow for the purpose of improving cut qualities, the following and other methods are known: a method of shielding an oxygen plasma with oxygen (see, for example, Japanese Patent Laid-Open No. 59-229282); a method of shielding a plasma gas flow with an inert gas to prevent the plasma gas flow from involving atmospheric air (see, for example, Japanese Patent Laid-Open No. 51-98652); and a method of shielding an oxygen plasma with hydrogen gas to cut stainless steel with an effect of achieving a high quality with metallic gloss and without oxidizing the stainless steel (see, for example, Japanese Patent Application No. 01-333548). The protective gas, for protecting the nozzle, is caused to flow so as to surround the plasma gas flow, whereby it can also serve as a shielding gas. However, in a case where expensive argon gas, hydrogen or a mixture of such gases is used to achieve a shielding effect as well as the protection of the nozzle, a problem in terms of operating cost is encountered, because a large amount of gas is consumed for the purpose of the protection of the nozzle while only a small amount of the gas can suffice for separating the plasma gas from the atmospheric air for the purpose of shielding.
With respect to the problem of the consumption of a large amount of the protective gas, conventional arts described below are known. First, a protective cap has an opening facing a nozzle orifice (an opening through which plasma gas flows) and a multiplicity of holes on the external side. By adopting such an arrangement, the flow rate of the protective gas flowing out from the opening is limited so that the influence upon the plasma gas flow is small even if the protective gas flow rate is high, thus preventing a reduction in cutting ability (see, for example, U.S. Pat. No. 4,861,962). However, this method is intended for use in the case of increasing the protective gas flow rate for the purpose of the protective gas cooling the nozzle exposed to heat at a high temperature. In this method, the protective gas is caused to flow at the time of cutting as well as at the time of piercing. Accordingly, the cooling effect of the protective gas on the plasma gas flow during cutting is not reduced, and a reduction in cut quality cannot be avoided.
As another conventional art, reducing the consumption of the oxygen gas used as a shielding gas, by causing a flow of the shielding gas only at the time of piercing and by shutting off the shielding gas during cutting, is known (see, for example, Japanese Utility Model Application No. 63-68879). However, this method is intended to prevent burning which can occur during cutting, and is different from those utilizing the shielding gas for protecting the nozzle from the dross. According to this method, therefore, the gas is caused to flow at a low rate at the time of piercing. Accordingly, the proportion of the shielding gas cost in the operating cost is small, but no protective gas effect is obtained since the shielding gas is shut off during cutting.
As still another conventional art, selectively using various active and inert gases with an inter-nozzle passage of a plasma cutting torch, having a multi-layer nozzle structure, to protect an electrode used in an inert gas atmosphere from an active gas, is known (see, for example, Japanese Patent Publication No. 51-16379). According to this method, the amount of gas used can be optimized if there are two or more kinds of gas. However, the torch is not provided with a nozzle protecting cap, and a protective gas cannot be used.
The present invention has been accomplished to overcome the above-described shortcomings of the conventional art, and an object of the present invention is to provide a plasma cutting method and a cutting apparatus in which a nozzle is suitably protected from dross, blown upwardly from a workpiece at the time of piercing and at the time of cutting, in which the cooling and disturbing effects of a protective gas on a plasma gas flow at the time of cutting are reduced to enable protection of the nozzle and good cutting, and which make it possible to reduce the operating cost of the protective gas.